Have you ever double clicked on the Application Menu icon in the upper left expecting the menu to rollout, only to see the application close?
This is more common now that the application menu icon is a little bigger “Big Red A” in AutoCAD 2009 and AutoCAD 2010 as well as other Autodesk products. I did this once while on the See the Difference with AutoCAD 2010 Tour.
Make sure to take your time when selecting the application menu icon and not get impatient and click it again causing a double click event and close the application. Patience is a virtue.